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SEP Implementation Mechanisms

SEP Implementation Mechanisms

Neighbor Negotiation
Mechanism

After an interface is added to a SEP segment,
neighbor negotiations start. The interface and its neighbor exchange
Hello packets to establish a neighbor relationship. After neighbor
negotiations succeed, the two interfaces continue to exchange Hello
packets to detect their neighbor status.

Neighbor negotiations
prevent unidirectional links because neighbor negotiations are bidirectional.
Interfaces at both ends of a link, must send Hello packets to each
other, as a means of status confirmation. If an interface does not
receive a Hello packet from an interface at the other end of a link
within a specified period, the interface considers the other to be
Down.

Synchronization of SEP
LSA Databases and Topology Display

Synchronization of SEP link state advertisement (LSA) databases

After neighbor negotiations are complete, devices in a SEP segment
enter the LSA database synchronization phase and periodically send
LSAs. After a device receives LSAs from other devices, the device
updates its LSA database. This ensures that the LSA databases of all
devices in the SEP segment are consistent.

If a device does
not receive LSAs from its peer device or other devices in the SEP
segment within three LSA transmission intervals, the device will age
the database that saves the LSAs of the other devices in the SEP segment.

When a faulty device in a SEP segment recovers, the device needs
to obtain topology information from the other devices in the SEP segment
and sends LSA request packets to the other devices. After receiving
LSA request packets from the device, neighboring interfaces reply
with LSA ACK packets that contain the latest link state information.

SEP segment topology display

The topology display function
allows you to view the topology with the highest network connectivity
on any device in a SEP segment. Link state synchronization ensures
that all devices in a SEP segment display the same topology.

Each interface in a SEP segment has a neighboring interface
in Up state and a brother interface, and each node has two interfaces
in the SEP segment.

If the primary edge interface is elected on a ring, the primary
edge interface is listed first in the topology information displayed
on each interface.

If the primary edge interface is not elected but the secondary
edge interface is elected, the secondary edge interface is listed
first in the topology information displayed on each interface.

Linear topology

All topologies except ring topologies are linear topologies.

For interfaces at both ends of a link:

If one interface functions as the primary edge interface, the
primary edge interface is listed first in the topology information
displayed on each interface.

If the primary edge interface is not elected but the secondary
edge interface is elected, the secondary edge interface is listed
first in the topology information displayed on each interface.

NOTE:

The constraints listed in Table 14-4 ensure
that each node in a ring or linear topology displays the same topology
information.

Primary Edge Interface
Election

Only interfaces that are configured as no-neighbor
edge interfaces, primary edge interfaces, and secondary edge interfaces
can participate in primary edge interface election.

NOTE:

If only one interface on a node has SEP enabled, you must set
the role of the interface to edge so that the interface can function
as an edge interface.

As shown in Figure 14-3, if there is no
faulty link on the network and SEP is enabled on the interfaces, the
following situations occur:

Common interfaces do not participate in primary edge interface
election. Only P1 on Router1 and P1 on Router5 participate in primary edge interface election.

If P1 on Router1 and P1 on Router5 have the same role, P1 with a higher MAC address is elected as the
primary edge interface.

After the primary edge interface is selected, it periodically
sends primary edge interface election packets without waiting for
the success of neighbor negotiations. A primary edge interface election
packet contains the interface role (primary edge interface, secondary
edge interface, or common interface), bridge MAC address of the interface,
interface ID, and integrity of the topology database.

Figure 14-3 Networking diagram of electing the primary edge interface

As shown in Figure 14-3, if a link fault
occurs in the SEP segment, P1 on Router1
and P1 on Router5 receive fault notification packets or P1 on
LSW5 does not receive primary edge interface election packets within
a specified period. Then P1 on Router1 becomes
the secondary edge interface. Consequently, two secondary edge interfaces
exist in the SEP segment and periodically send primary edge interface
election packets.

When all link faults in the SEP segment are
rectified, the two secondary edge interfaces can receive primary edge
interface election packets and elect a new primary edge interface
within a configured interval (1s by default).

Specifying an Interface
to Block

Normally, a blocked interface is one of the two
interfaces that complete neighbor negotiations last. In some cases,
however, the negotiated blocked interface may not be the required
one. You can specify an interface to block according to network requirements.
The specified interface preempts to be the blocked interface only
after the preemption mechanism takes effect.

Specify the interface in the middle of a SEP segment as
the blocked interface.

-

Specify a blocked interface based on the configured hop
count.

SEP sets the hop count of the primary edge interface to
1 and the hop count of the neighboring interface of the primary interface
to 2. Hop counts of other interfaces increase by steps of 1 in the
downstream direction of the primary edge interface.

Specify a blocked interface based on the device and interface
names.

After SEP is configured, the interface to be blocked is
determined by the device and interface names. Before specifying an
interface to block, run the display command to view the current
ring topology and all interfaces, and then specify the device and
interface names.

If multiple interfaces on the ring have the
same device and interface names, SEP blocks the interface nearest
to the primary edge interface in the topology.

NOTE:

If you change the device name or interface name after specifying
the interface to block, the interface cannot preempt to be the blocked
interface.

Preemption

After the interface blocking mode is specified,
whether a specified interface will be blocked is determined by the
preemption mode. Table 14-6 lists the preemption
modes.

Table 14-6 Preemption mode

Preemption Mode

Description

Non-preemption mode

When all link faults are rectified or the last two interfaces
enabled with SEP complete neighbor negotiations, interfaces send blocking
status packets to each other. The interface with the highest priority
is then blocked, and the other interfaces enter the Forwarding state.

Preemption Mode

NOTE:

Preemption can only
be implemented on the device where the primary edge interface or no-neighbor
primary edge interface resides.

Preemption is classified into delayed preemption and manual
preemption.

Delayed preemption

After all the faulty interfaces recover,
the edge interfaces no longer receive fault notification packets.
If the primary edge interface does not receive fault advertisement
packets within 3 seconds, it starts the delay timer. After the delay
timer expires, nodes in the SEP segment start blocked interface preemption.

Manual preemption

When the link status databases of the
primary edge interface and secondary edge interface are complete,
the primary edge interface or brother interface of the no-neighbor
primary edge interface sends preemption packets to block a specified
interface. The specified interface then sends blocking status packets
to request the previously blocked interface to transition to the Forwarding
state.

NOTE:

Only two interfaces on a device can
be added to the same SEP segment. If one interface is the no-neighbor
primary edge interface, the other interface is the brother interface
of the no-neighbor primary edge interface.

Whether the brother
interface of the no-neighbor primary edge interface needs to send
preemption packets depends on whether the brother interface is blocked.

If the brother interface is blocked, it does not need to send
preemption packets.

If the brother interface is unblocked, it needs to send preemption
packets.

SEP Topology Change
Notification

SEP considers that the topology of a SEP-enabled
network changes in either of the following situations described in Table 14-7.

An interface
fault can be a link fault or neighboring interface fault.

If
a device having an interface in Forwarding state in the SEP segment
receives a fault advertisement packet, the device needs to send a
Flush-Forwarding Database (Flush-FDB) packet through the interface
to notify other nodes in the SEP segment that there is a change in
topology.

The fault is rectified and the preemption function takes
effect.

After faults occur in the SEP segment and the last faulty
interface recovers, the blocked interface is preempted and the topology
is considered changed.

Preemption is triggered by the primary
edge interface. When an interface in a SEP segment receives a preemption
packet from the primary edge interface, the interface needs to send
Flush-FDB packets to notify other nodes in the SEP segment that there
is a change in topology.

Figure 14-4 Networking diagram for SEP topology change notification

NOTE:

The topology change notification function
is configured on devices that connect an upper-layer network and a
lower-layer network. If the topology of one network changes, devices
affected inform the other network of the change.

Table 14-8 lists
the scenarios in which topology changes are reported.

Table 14-8 SEP topology change notification

SEP Topology Change Notification

Scenario

Description

Solution

Topology change notification from a lower-layer network
to an upper-layer network

A SEP network is connected to an upper-layer network running
other features such as SEP and STP.

If the blocked interface on a lower-layer SEP network is manually
changed, the topology of the SEP segment changes. Because the upper-layer
network is unable to detect the change in topology, traffic is interrupted.

If an interface on a lower-layer SEP network becomes faulty, the
topology of the SEP segment changes but the upper-layer network is
unable to detect the change. As a result, traffic is interrupted.

Configure the SEP topology change notification function.

Suppression of SEP TC
Notification Packets

Topology changes of a SEP segment are
advertised to other SEP segments or upper-layer networks. A large
number of topology change (TC) notification packets are generated
in the following cases:

A link becomes disconnected transiently.

A SEP segment is attacked by invalid TC notification packets.

There are multiple SEP ring networks.

Figure 14-5 shows a networking
scenario with three SEP ring networks. If the topology of SEP segment
3 changes, the number of TC notification packets doubles and SEP segment
2 is flooded with these packets. Each time TC notification packets
pass through a SEP segment, the number of TC notification packets
doubles.

Figure 14-5 Networking diagram for multiple SEP ring networks

Sending a large number of TC notification packets reduces
the CPU capability to quickly process other types of packets. In addition,
devices in SEP segments frequently update MAC address entries, heavily
consuming bandwidth resources. To solve such problems, the following
measures can be taken to suppress TC notification packets:

Configure a device to process only one of the TC notification
packets carrying the same source address.

Configure a device to process a specified number of TC notification
packets within a specified period. By default, three TC notification
packets with different source addresses are processed in 2s.

Avoid the networking scenario having more than three SEP ring
networks.

SEP Multi-Instance

In common SEP networking shown in Figure 14-6, a physical ring
network can be configured with only one SEP segment in which only
one interface can be blocked.

If an interface in a complete
SEP segment is blocked, all service data is transmitted only along
the path where the primary edge interface is located. The path where
the secondary edge interface is located remains idle, wasting bandwidth.

Figure 14-6 Networking diagram for SEP

SEP multi-instance allows two SEP segments
to be configured on a physical ring. Each SEP segment independently
detects the completeness of the physical ring, blocks or unblocks
interfaces without affecting the other.

A physical
ring may contain one or two SEP segments. Each SEP segment needs to
be configured with a protected instance, each protected instance indicating
a VLAN range. The topology calculated by a SEP segment is only valid
for that SEP segment.

After different protected instances
are configured for SEP segments and the mapping between protected
instances and VLANs is set, a blocked interface is only valid for
the VLANs protected by the SEP segment where the blocked interface
resides. Data traffic for different VLANs can be transmitted along
different paths. This implements traffic load balancing and link backup.

Figure 14-7 Networking diagram for SEP multi-instance

As shown in Figure 14-7, the SEP multi-instance
ring network that consists of Router1 to Router4 has two SEP segments. P1 is the blocked interface in SEP segment
1, and P2 is the blocked interface in SEP segment 2.

Protected instance 1 is configured in SEP segment 1 to protect
the data from VLAN 100 to VLAN 200. The data is transmitted along
path Router1->Router2. As the blocked
interface in SEP segment 2, P2 blocks only the data from VLAN 201
to VLAN 400.

Protected instance 2 is configured in SEP segment 2 to protect
the data from VLAN 201 to VLAN 400. The data is transmitted along
path Router3->Router4. As the blocked
interface in SEP segment 1, P1 blocks only the data from VLAN 100
to VLAN 200.

When a node fault or link fault occurs, each SEP segment
calculates its own topology independently, and the nodes in each SEP
segment update their own LSA databases.

As shown in Figure 14-8,
a fault occurs on the link between LSW3 and LSW4. The link fault does
not affect the transmission path for the data from VLAN 100 to VLAN
200 in SEP segment 1, but blocks the transmission path for the data
from VLAN 201 to VLAN 400 in SEP segment 2.

Figure 14-8 Networking diagram for a link fault on a SEP multi-instance
network

After the link between Router3 and Router4 becomes faulty, Router3 starts to send LSAs
to instruct the other devices in SEP segment 2 to update their LSA
databases, and the blocked interface enters the Forwarding state.
After the topology of SEP segment 2 is recalculated, the data from
VLAN 201 to VLAN 400 is transmitted along path Router3->Router1->Router2.

After the link between Router3 and Router4 recovers,
the devices in SEP segment 2 perform delayed preemption. After the
preemption delay expires, P1 becomes the blocked interface again,
and sends LSAs to instruct the other devices in SEP segment 2 to update
their LSA databases. After the topology of SEP segment 2 is recalculated,
the data from VLAN 201 to VLAN 400 is transmitted along path Router3->Router4.